BEIRUT, Lebanon — Scores of men, women and children were killed outside Damascus on Wednesday in an attack marked by the telltale signs of chemical weapons: row after row of corpses without visible injury; hospitals flooded with victims, gasping for breath, trembling and staring ahead languidly; images of a gray cloud bursting over a neighborhood.

But even with videos, witness accounts and testimonies by emergency medics, it was impossible to say for certain how many people had been killed and what exactly had killed them. The rebels blamed the government, the government denied involvement and Russia accused the rebels of staging the attack to implicate President Bashar al-Assad’s government.

Images of death and chaos poured out of Syria after what may be the single deadliest attack in more than two years of civil war. Videos posted online showed dozens of lifeless bodies, men wrapped in burial shrouds and children, some still in diapers. There were hospital scenes of corpses and the stricken sprawled on gurneys and tile floors as medics struggled to resuscitate them.

Getting to the bottom of the assault could well alter the course of the conflict and affect the level of the West’s involvement.

President Obama said almost exactly a year ago that the use of chemical weapons was a red line. But the subsequent conclusion by the White House that the Syrian Army had used chemical weapons did not bring about a marked shift in American engagement.

This latest attack, by far the largest chemical strike yet alleged, could tip that balance — as many foes of Mr. Assad hope it will.

But like so much in Syria, where the government bars most reporters from working and the opposition heavily filters the information it lets out, the truth remains elusive.

The attack was especially conspicuous given the presence in Damascus of a team sent by the United Nations to investigate chemical strikes reportedly waged earlier in the war. The United States, the European Union and other world powers called for the investigators to visit the site of Wednesday’s attack.

The Security Council, meeting in emergency session, issued a statement calling for a prompt investigation of the allegations and a cease-fire in the conflict, but took no further action.

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Survivors from what activists say was a chemical weapons attack at a mosque on Wednesday in the Duma neighborhood of Damascus.CreditBassam Khabieh/Reuters

“I can say that there is a strong concern among Council members about the allegations and a general sense that there must be clarity on what happened, and that the situation has to be followed carefully,” said María Cristina Perceval of Argentina, the president of the Council, after the meeting. “All Council members agreed that any use of chemical weapons, by any side under any circumstances, is a violation of international law.”

The ranking diplomat from Britain, Philip Parham, told reporters later outside the Security Council chambers that representatives of at least 35 countries had signed a letter to Secretary General Ban Ki-moon requesting that Syrian authorities grant the United Nations investigative panel in Syria “urgent access” to the attack site.

But Mr. Parham declined to specify the signatories or to divulge whether any of the 15 Security Council members had proposed any stronger measures during their closed-door consultations.

In the opposition’s account of the deadly events, Mr. Assad’s forces deployed poison gas on a number of rebel-held suburbs east of Damascus, the capital. They described medics finding people dead in their homes.

Videos posted online showed mostly men and children, but the opposition activists said that many women were killed too, but that out of respect they were not photographed.

The actual death toll remained unclear. The Syrian Observatory for Human Rights said late Wednesday that more than 130 people had been confirmed dead in attacks around Damascus, though it could not confirm the use of gas. Other opposition estimates put the death toll at more than 1,000.

“I saw many children lying on beds as if they were sleeping, but unfortunately they were dead,” said an activist reached via Skype in the suburb of Erbin, who gave his name as Abu Yassin.

“We thought this regime would not use chemical weapons, at least these days with the presence of the U.N. inspectors,” he said. “It is reckless. The regime is saying, ‘I don’t care.’ ”

Others said that field hospitals were overwhelmed with the number of patients and that many ran out of medication. An activist who gave only his first name, Mohammed, said the dead in one suburb, Zamalka, were laid out in front of a mosque, where a voice over loudspeakers called on residents to identify their relatives.

The video record posted online did not provide enough detail to draw a complete picture of what happened. Unlike the videos often uploaded by the opposition, the images on Wednesday did not show the immediate aftermath of the attacks in the communities.

The videos, experts said, also did not prove the use of chemical weapons, which interfere with the nervous system and can cause defecation, vomiting, intense salivation and tremors. Only some of those symptoms were visible in some patients.

Gwyn Winfield, editor of CBRNe World, a journal that covers unconventional weapons, said that the medics would most likely have been sickened by exposure to so many people dosed with chemical weapons — a phenomenon not seen in the videos. He said that the victims could have been killed by tear gas used in a confined space, or by a diluted form of a more powerful chemical agent. Others suggested that toxic industrial chemicals might have been used.

Some witness testimony suggested that residents, used to seeking cover from government shelling and airstrikes by running into underground shelters, had made the situation worse. In one video, a young medic said that residents had hidden in their basements, where the gas collected and suffocated them.

“The descent of the citizens into the basements increased the number of wounded and the number of martyrs,” the medic said, before breaking into tears and adding that many from the medical corps also succumbed to the gases.

It was not clear whether the team sent to Syria by the United Nations would be able to investigate the new reported attacks. The team arrived Sunday after months of negotiations with the Syrian government and is authorized to visit only three predetermined sites.

The White House said that Syria should provide access to the United Nations, and that those found to have used chemical weapons should be held accountable. Other countries, including Britain and France, offered similar expressions of concern.

Russia wrote off the attack as a “preplanned provocation” orchestrated by the rebels and said they had launched the gas with a homemade rocket from an area they controlled.

“All of this looks like an attempt at all costs to create a pretext for demanding that the U.N. Security Council side with opponents of the regime and undermine the chances of convening the Geneva conference,” said the statement, issued by Aleksandr Lukashevich, a spokesman for the Foreign Ministry. He also called for a “professional and fair investigation.”

At least one photograph posted on Facebook by an activist showed what looked like a makeshift rocket. But loyalist militias and Hezbollah have both fired makeshift rockets at rebel positions in this war, and could presumably be suspects for any attacks with improvised rockets on rebel-controlled neighborhoods.

The Syrian Army, in a statement read on state television, denied having used chemical weapons, calling the accusations part of a “filthy media war” in favor of the rebels. The claims “are nothing but a desperate effort to cover their defeat on the ground, and reflect the state of hysteria, confusion and collapse of these gangs and those who support them,” the statement said.

Louay Mekdad, a media coordinator for the military wing of the opposition Syrian National Council, said the attack showed that Mr. Assad “doesn’t care any longer about red lines since he has already exceeded too many of them while the world has showed no reaction.”

Mr. Mekdad called on the Security Council and international powers to “live up to their moral and historic responsibility” to protect civilians in Syria. “If the international community doesn’t move now, when is it going to move?” he asked.

Reporting was contributed by David M. Herszenhorn from Moscow; C.J. Chivers from the United States; Peter Baker and Thom Shanker from Washington; Alan Cowell from London; Alissa J. Rubin from Paris; Mac Bishop from New York; and Karam Shoumali from Antakya, Turkey.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Scores Killed in Syria, With Signs of Chemical War. Order Reprints | Today’s Paper | Subscribe